System and method for mitigating four-wave-mixing effects

ABSTRACT

In one embodiment, a system includes at least one tone generator, a first transmitter, and a second transmitter. The at least one tone generator is operable to generate a plurality of modulation tones comprising at least a first modulation tone having a first tone frequency and a second modulation tone having a second tone frequency that is different from the first tone frequency. The first transmitter is operable to apply the first modulation tone to a first optical signal such that at least a portion of the first optical signal is divided into one or more sidebands. The second transmitter is operable to apply the second modulation tone to a second optical signal such that at least a portion of the second optical signal is divided into one or more sidebands.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.13/086,219 filed Apr. 13, 2011 and entitled “System and Method forMitigating Four-Wave-Mixing Effects”.

TECHNICAL FIELD

The present disclosure relates generally to optical networking, and morespecifically to a system and method for mitigating four-wave-mixingeffects.

BACKGROUND

Fiber-optic communication may involve transmitting information from oneplace to another by sending optical signals through an optical fiber. Anoptical fiber may include any material that facilitates transmission ofoptical signals, such as an optical waveguide. An optical signal mayform an electromagnetic carrier wave modulated to carry information.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example transmitter for transporting signals overa fiber optic network;

FIG. 2 illustrates an example system for transporting signals over afiber optic network;

FIG. 3 illustrates a table containing an example set of frequencyvariables for the system of FIG. 2;

FIG. 4 illustrates an example plot of two optical signals and their FWMproducts;

FIG. 5 illustrates an example plot of an optical signal after a phasemodulation tone has been applied;

FIG. 6A shows an example plot of channels A-H and their four-wave mixingproducts according to one embodiment;

FIG. 6B shows an example plot of the channels A-H of FIG. 6A and theirfour-wave mixing products that are have frequencies within 25 GHz ofchannels A-H according to one embodiment;

FIG. 7A illustrates an example plot of the noise floor characteristicswhen every channel in an example channel window has a modulation tone ofthe same frequency;

FIG. 7B illustrates an example plot of the noise floor characteristicsof FIG. 7A when the modulation tones have different frequencies;

FIG. 8 illustrates an example plot of the noise spectrum within 600 MHzof channel B of FIGS. 7A and 7B for three difference scenarios;

FIG. 9 illustrates an example method for transporting signals over afiber optic network according to one embodiment.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Overview

In one embodiment, a system includes at least one tone generator, afirst transmitter, and a second transmitter. The at least one tonegenerator is operable to generate a plurality of modulation tonescomprising at least a first modulation tone having a first tonefrequency and a second modulation tone having a second tone frequencythat is different from the first tone frequency. The first transmitteris operable to apply the first modulation tone to a first optical signalsuch that at least a portion of the first optical signal is divided intoone or more sidebands. The second transmitter is operable to apply thesecond modulation tone to a second optical signal such that at least aportion of the second optical signal is divided into one or moresidebands.

Description

Fiber-optic communication may involve transmitting information from oneplace to another by sending optical signals through an optical fiber. Anoptical fiber may include any material that facilitates transmission ofoptical signals, such as an optical waveguide. An optical signal mayform an electromagnetic carrier wave modulated to carry information. Theprocess of communicating using fiber optics may include creating theoptical signal using a transmitter, relaying the optical signal alongthe fiber, and converting the optical signal into an electrical signal.

In a wavelength-division-multiplexing (WDM) architecture, signals arecarried by multiple colors of light (different optical wavelengths)within the same optical fiber so that transmission bandwidth can beincreased without necessarily deploying new fiber. A WDM system uses amultiplexer at the transmitter to join signals together and ademultiplexer at the receiver to split the signals apart. Densewavelength division multiplexing (DWDM) may use the same channel windowas WDM but with denser channel spacing.

A channel window is a band of wavelengths that may contain one or moreoptical signals, or channels. Examples of channel windows may include,but are not limited to, the original band (“O band”), ranging from 1260to 1360 nanometers; the extended band (“E band”), ranging from 1360 to1460 nanometers; the short band (“S band”), ranging from 1460 to 1530nanometers; the conventional band (“C band”), ranging from 1530 to 1565nanometers; the long band (“L band”), ranging from 1565 to 1625nanometers; and the ultralong band (“U band”), ranging from 1625 to 1675nanometers.

In some circumstances, the optical signal may become weak or distortedduring transmission through the fiber. For example, nonlinearities existthat may affect how information is transmitted through a fiber. Thesenonlinearities may include stimulated Raman scattering (SRS),cross-phase modulation (XPM), four wave mixing (FWM), and stimulatedBrillouin scattering (SBS).

The SRS effect may cause optical energy to be transferred between a pairof optical channels. Within the erbium-doped fiber amplifier (EDIFA)spectrum, crosstalk may increase as the separation between two opticalsignal channels increases. The SRS crosstalk degradescomposite-second-order (CSO) distortion of the analog channels andmodulation error rate (MER) of the quadrature amplitude modulation (QAM)channels. At a given link loss budget, the SRS effect my limit how widethe channel window can be and how many channels can be carried withinthe channel window.

The XPM effect occurs when the optical index of fiber experienced by onesignal wavelength is modulated by the intensity fluctuation of anotherwavelength. When two optical channels are too close to each other, XPMmay impose crosstalk at high signal frequency QAM band and result in MERor even bit error rate (BER) degradation. XPM crosstalk may confineitself in multiple service operator (MSO) distribution networks with 100GHz channel spacing.

FWM may occur when multiple wavelengths travel in the same direction inthe fiber, producing new wavelengths as a result of fiber properties.These new wavelengths, referred to as FWM products, raise two primaryconcerns: the location of the FWM products and their power. The power ofFWM products depends on both the power in generating the opticalwavelengths and on how well the optical wavelengths stay phase matched.The closer the optical wavelengths are to the zero dispersion wavelength(the wavelength at which there is no dispersion in the fiber), the moreeasily they stay phase matched. In standard single mode fiber, the zerodispersion point may be around 1311 nanometers, but may vary anywherefrom 1301 to 1321 nanometers in some circumstances.

The location of the FWM products may be determined by calculating thefrequency of the FWM products as a function of the frequencies of thecontributing signals. For example, the frequency of FWM productsgenerated from two optical channels may be equal to:f _(fwm)=2·f ₁ −f ₂

The frequency of FWM products generated from three optical channels maybe equal to:f _(fwm) =f ₁ +f ₂ −f ₃

In some circumstances, the FWM effect may produce FWM products that landnear the optical channels. For example, if a fiber is carrying fouroptical channels numbered 1-4, the FWM products resulting from channels2 and 3 may have frequencies near channels 1 and 4.

The SBS nonlinearity results from interactions of light waves (photon)and sound (phonon) in the fiber. Without an SBS remedy, the maximumfiber launch power per wavelength may be around 6 to 7 dBm in somecircumstances.

Two examples of SBS remedies are dither signals and modulation tones.Dither signals may be applied to a laser transmitter to broaden theeffective linewidth of the laser. Dithering signals may be applied, forexample, by modulating the laser transmitter's bias input. Although thefinite linewidth of the laser, which is a function of the fixed physicalcharacteristics of the laser, may actually remain constant, thedithering of the laser bias current may cause the wavelength of thelaser's output signal to be cyclically shifted about the natural outputwavelength produced by the un-dithered laser. This effectively spreadsthe output linewidth over a range centered about the un-dithered output.Dithering the laser transmitter's optical center frequency may allow thepower limit to be increased.

Modulation tones may divide optical signal into one or more sidebands.Examples of modulation tones may include phase modulation tones andamplitude modulation tones. In some embodiments, modulation tones maysuppress SBS effects. For example, the sidebands resulting fromapplication of modulation tones may be sufficiently separate to haveindependent power thresholds at which SBS interference occurs. Bydistributing the optical signal across one or more sidebands, theoverall power transmitted across the optical fiber can be increasedwithout producing deleterious interference. Interference may includeeffects from distortions, carriers, spurious products, noise, and othereffects. Modulation tones that suppress SBS effects may also be known asSBS suppression tones.

In some embodiments, the frequency of modulation tones may beconsiderably higher than the frequency of dither signals. For example,in some embodiments, modulation tones may have a frequency higher thanthe frequency of the information band carried by the optical channels,and dither tones may have a frequency lower than the frequency of theinformation band carried by the optical channels. For example,modulation tones may have a frequency equal to or higher than double thefrequency of the optical channels in some embodiments, whereas dithersignals may have a frequency in the audio frequency band (e.g.,approximately between 30 Hz to 20 KHz).

The presence of modulation tones, however, may result in additional FWMproducts. If these FWM products fall between 1 to 5 GHz from the signalwavelength, the 2 GHz and 4 GHz and other harmonics may fold the opticalbeating noise back to the 1 GHz CATV band, and therefore imposecarrier-to-noise ratio (CNR) and MER degradation onto RF channels.Teachings of certain embodiments recognize the capability to reduce theability of FWM products of modulation tones to interfere with theoptical signal channels.

FIG. 1 illustrates an example transmitter 100 for transporting signalsover a fiber optic network. Transmitter 100 comprises a laser source 110and an optical modulator 120. In operation, according to one embodiment,transmitter 100 may receive signals from tone generator 130, dithergenerator 140, and information input 150 to produce a modulated opticalsignal 160.

Light source 110 may include any device capable of generating and/ortransmitting light. Examples of light source 110 may include, but arenot limited to, light-emitting diodes and laser diodes. Examples oflaser diode light sources may include, but are not limited to, avertical cavity surface emitting laser and a distributed feed backlaser.

Optical modulator 120 may include any device capable of modulating thelight output of light source 110. Modulation may refer to the process ofvarying one or more properties of a carrier waveform with respect to amodulating signal. Examples of an optical modulator 120 may include, butare not limited to, an electro-optic modulator, an electro-absorptionmodulator, and a Mach-Zehnder interferometer.

Tone generator 130 may include any device capable of generating amodulation tone. Modulation tones may divide an optical signal into oneor more sidebands. In some embodiments, these sidebands may besufficiently separate to have independent power thresholds at which SBSinterference occurs. By distributing the optical signal across one ormore sidebands, the overall power transmitted across the optical fibermay be increased without producing deleterious interference.

Dither generator 140 may include any device capable of generating adithering signal. The dithering signal broadens the light energyprovided by light source 110. For example, if light source 110 is alaser, dither generator 140 may provide a dithering signal that spreadsthe linewidith of the light provided by light source 110. Dithering thelaser's optical center frequency may allow the power limit to beincreased.

Information input 150 may include any source of signals to be modulatedinto an optical signal. Examples of signals from information input 150may include, but is not limited to, voice, data, and video signals. Inone example embodiment, information input 150 provides radio frequency(RF) signals, such as RF signals representing cable television channels.

In operation, according to one embodiment, light source 110 may generateor transmit electromagnetic energy. Tone generator 130 may introduce amodulation tone to the electromagnetic energy to divide theelectromagnetic signal into one or more sidebands. Dither generator 140may introduce a dithering signal that spreads the effective linewidth ofthe electromagnetic signal provided by light source 110. Opticalmodulator 120 may modulate the electromagnetic signal provided by lightsource 110 with respect to information signals provided by informationinput 150. Optical modulator 120 transmits the modulated electromagneticsignal as modulated optical signal 160.

FIG. 2 illustrates an example system 200 for transporting signals over afiber optic network. System 200 comprises transmitters 210, a wavelengthmultiplexer 220, an optical fiber 230, a wavelength de-multiplexer 240,and receivers 250.

Transmitters 210 may include any device capable of generating ortransmitting modulated optical signals. One example of a transmitter 210may include transmitter 100 of FIG. 1. For example, transmitters 210 mayreceive signals from a tone generator such as tone generator 130, adither generator such as dither generator 140, and an information inputsuch as information input 150. Transmitters 210 may also generatemodulated optical signals such as modulated optical signal 160.

In some embodiments, each transmitter 210 provides different modulatedoptical signals. For example, each modulated optical signal mayrepresent a different channel with a different carrier frequency. In theexample of FIG. 2, eight transmitters 210 a-210 h provide eightdifferent modulated optical signals to wavelength multiplexer 220.Different embodiments of system 200 may include more or fewertransmitters 210. Transmitters 210 may carry the same or differentinformation.

Wavelength multiplexer 220 may include any device capable ofmultiplexing optical carrier signals into a single optical fiber 230 byusing different wavelengths of laser light. Wavelength multiplexer 220has inputs corresponding to each transmitter 210 and at least one outputcorresponding to optical fiber 230. The outputs of transmitters 210 areoperatively connected with respect to one of the inputs of wavelengthmultiplexer 220. The output of wavelength multiplexer 220 is configuredto transmit a combined optical signal comprised of the transmitteroptical signals having respective transmitted power levels. In someembodiments, wavelength multiplexer 220 can be implemented by a cascadeof multiplexers, each with more or fewer inputs than needed.

Optical fiber 230 may include any material that facilitates transmissionof optical signals, such as an optical waveguide. Although FIG. 2 showsone optical fiber 230 between wavelength multiplexer 220 and wavelengthde-multiplexer 240, system 200 may also include numerous discretepassive and active components such as fiber optic cable, terminations,splitters, amplifiers, WDMs, etc. In some embodiments, optical fiber 230may be part of a hybrid-fiber coaxial (HFC) network.

Wavelength de-multiplexer 240 may include any device capable ofdemultiplexing multiple optical carrier signals from optical fiber 230.Wavelength de-multiplexer 240 may have at least one input correspondingto optical fiber 230 and outputs corresponding to each receiver 250. Theoutput of optical fiber 230 is operatively connected with respect to theinput of wavelength de-multiplexer 240. The outputs of wavelengthde-multiplexer 240 are configured to transmit optical signals to each ofreceivers 250. In some embodiments, wavelength de-multiplexer 240 can beimplemented by a cascade of demultiplexers, each with more or feweroutputs than needed. Receivers 250 may include any device capable ofreceiving modulated optical signals from wavelength de-multiplexer 240.

In operation, according to one embodiment, each transmitter 210 providesa modulated optical signal to wavelength multiplexer 220. Wavelengthmultiplexer 220 multiplexes the modulated optical signals into singleoptical fiber 230 by using different wavelengths of laser light. Themodulated optical signals travel through single optical fiber 230 towavelength de-multiplexer 240. Wavelength de-multiplexer 240demultiplexes the modulated optical signals from optical fiber 230 andtransmits them to receivers 250.

FIG. 3 illustrates a table 300 containing an example set of frequencyvariables for system 200. Table 300 lists frequencies for channels A-Hcorresponding to transmitters 210 a-210 h of FIG. 2. Each channel may becharacterized by a channel frequency (v), a modulation tone frequency(f), and a dither frequency (d). Selection of each channel frequency,modulation frequency, and/or dither frequency may affect the existenceand impact of nonlinearities such as SRS, XPM, FWM, and SBS.

As one example, selection of each channel frequency may affect thelocation of FWM products. FIG. 4 illustrates an example plot 400 of twooptical signals and their FWM products. In this example, channels A and13 have FWM products with frequencies equal to 2v_(A)−v_(B) and2v_(B)−v_(A). As shown in plot 400, the difference between v_(A) andv_(B) is approximately equal to the difference between v_(A) and2v_(A)−v_(B) and to the difference between v_(B) and 2v_(B)−v_(A).

In some examples, channels A-H may be equally spaced. For example, thedifference between v_(A) and v_(B) for adjacent channels A and B may beapproximately equal to the distance between between v_(B) and v_(C) foradjacent channels B and C. If so, then FWM product 2v_(B)−v_(A) may benear or equal to v_(C) for channel C, which may result in interferencein channel C. As another example, a FWM product may have a frequencyequal to v_(A)+v_(C)−v_(B), which may be near or equal to v_(B) forchannel B and result in interference in channel B.

Accordingly, teachings of certain embodiments recognize the ability toselect channel frequencies such that the channels are unequally spaced.In some embodiments, all channels may be unequally spaced. In otherembodiments, only some channels are unequally spaced. For example, insome embodiments channels A-H may be organized into subsets such assubset A-D and subset E-H. In this example, all the channels withinsubset A-D may be unequally spaced and all the channels within subsetE-H may be unequally spaced. In one example embodiment, the differencebetween channels v_(A) and v_(B) may be equal to the difference betweenchannels v_(E) and v_(F), the difference between v_(B) and v_(C) may beequal to the difference between v_(F) and v_(G), and the differencev_(C) and v_(D) may be equal to the difference between v_(G) and v_(H).

As another example of how selection of frequencies may affect theexistence and impact of nonlinearities, the introduction of modulationtones may allow the overall power transmitted across the optical fiberto be increased. FIG. 5 illustrates an example plot 500 of an opticalspectrum after a modulation tone has been applied. In this example,applying a modulation tone to channel A has resulted in multiplesidebands. The spacing of each sideband may be based on the frequency ofthe modulation tone. For example, applying a modulation tone of 2.00 GHzmay result in sidebands offset at ±2 GHz, ±4 GHz, −6 GHz, etc.

In plot 500, the sidebands at ±2 GHz are almost as powerful as theprimary signal, the sidebands at ±4 GHz are less powerful, and the powerof the sidebands at ±6 GHz is insignificant. The number of significantsidebands may affect FWM crosstalk. For example, consider aconfiguration of system 200 where FWM products are 25 GHz away fromchannel C, but each of channels A-H has significant sidebands to ±6 GHz.This may result in FWM products of the sidebands at 1 GHz away from the6 GHz sideband of channel B. With aging and temperature variation,transmitters 210 may shift in frequency such that the FWM products ofthe sidebands shift even closer to channel B and impair the signal ofchannel B.

Accordingly, teachings of certain embodiments recognize the capabilityto set the modulation tones at different frequencies on differentchannels. For example, the frequencies of the modulation tones f_(A),f_(B), and f_(C) for channels A, B, and C may be set at 1.975, 2.000,and 2.025 GHz respectively, instead of setting all of them to 2.000 GHz.In this example, FWM crosstalk may still fall near 1 GHz but may bespread out over a range of frequencies (e.g., 900 MHz to 1100 MHz) suchthat the power spectral density is reduced. Teachings of certainembodiments recognize that reducing the power spectral density mayreduce FWM crosstalk power in the information signal pass band.

In some embodiments, every modulation tone may have a differentfrequency. In other embodiments, some modulation tones may remain thesame. For example, in one embodiment, adjacent channels may havemodulation tones of different frequencies, whereas non-adjacent channelsmay have modulation tones of the same frequency. For example, thefrequency of the modulation tone for channel A may be different fromchannel B but the same as channel C, and the frequency of the modulationtone for channel B may be different from channel C but the same aschannel D.

In another example embodiment, the modulation tone frequencies may bedifferent for every channel within a subset of channels. For example, insome embodiments channels A-H may be organized into subsets such assubset A-D and subset E-H. In this example, all the channels withinsubset A-D may have modulation tones of different frequencies, and allmodulation tones within subset E-F may have modulation tones ofdifferent frequencies. In one example embodiment, channels A and E mayhave modulation tones of the same frequency, B and F may have modulationtones of the same frequency, C and G may have modulation tones of thesame frequency, and D and H may have modulation tones of the samefrequency. In another example embodiment, channels A and E may havemodulation tones of the same frequency, B and G may have modulationtones of the same frequency, D and H may have modulation tones of thesame frequency, and C and F may have modulation tones of differentfrequencies.

In some embodiments, modulation tone frequencies may be selected basedon the relationship between channels and their FWM products. FIG. 6Ashows an example plot 600A of channels A-H and their four-wave mixingproducts according to one embodiment. In plot 600A, no modulation toneshave been applied to channels A-H.

In plot 600A, the origin of the x-axis is defined as the frequency ofchannel A, and all other frequencies are measured along the x-axis asbeing offset from channel A. For example, in plot 600A, channels B-H aredefined in relation to channel A as shown in Table 1, below:

Channel Frequency A υ_(A) B υ_(A) + 175 GHz C υ_(A) + 300 GHz D υ_(A) +500 GHz E υ_(A) + 725 GHz F υ_(A) + 875 GHz G υ_(A) + 1125 GHz  Hυ_(A) + 1400 GHz 

Transmitting channels A-H through an optical network may result in bothtwo-tone and three-tone FWM products. Two-tone FWM products includethose FWM products receiving contributions from two channels. Thefrequency of a two-tone FWM product may be equal to 2v₂−v₁. Three-toneFWM products include those FWM products receiving contributions fromthree channels. The frequency of a three-tone FWM product may be equalto v₁+v₃−v₂.

In the example of plot 600A, the frequencies of channels A-H are definedsuch that FWM products are generally no closer than 25 GHz away from achannel. As transmitters age, channels A-H may experience wavelengthdrifting and result in shifts on the wavelength of some FWM products. Inthese circumstances, those FWM products closest to the channels will beamong the most likely FWM products to drift into the optical channel andresult in interference. FIG. 6B shows an example plot 600B of thechannels A-H of FIG. 6A and their four-wave mixing products that arehave frequencies within 25 GHz of channels A-H according to oneembodiment.

Teachings of certain embodiments recognize the capability to applymodulation tones to channels A-H so as to reduce the ability of a FWMproduct near an optical channel to interfere with that optical channel.In one example embodiment, modulation tones are applied such that, forat least some two-tone FWM products closest to the optical channels, thechannels contributing to the two-tone FWM products and the channelsclosest to the two-tone FWM products have modulation tones of differentfrequencies. For example, Table 2, below, lists the two-tone FWMproducts, the channels contributing to the two-tone FWM products, thechannels closest to the two-tone FWM products, and the crosstalk powerof the two-tone FWM products.

Channel Closest FWM Channel 2 Channel 1 Two-tone FWM Product to FWMCrosstalk (ν₂) (υ₁) Frequency (2ν₂ − υ₁) Product (dB) D C υ_(A) + 700GHz E −68.4 D E υ_(A) + 275 GHz C −71.9 E C υ_(A) + 1150 GHz  G −81.2 EG υ_(A) + 325 GHz C −82.2 G F υ_(A) + 1375 GHz  H −71.3 G H υ_(A) + 850GHz F −73.0

In this example, modulation tone frequencies may be selected such thatchannels C, D, and E all have modulation tones of different frequencies;channels C, E, and G all have modulation tones of different frequencies;and channels F, G, and H all have modulation tones of differentfrequencies. Such an arrangement may be accomplished by applying fivedifferent modulation tones to channels A-H, as listed below in Table 3.

Modulation Tone Channel Example Value 1 C f₁ 2 D, H f₁ + 28 MHz 3 A, Ef₁ + 58 MHz 4 B, G f₁ + 88 MHz 5 F f₁ + 118 MHz 

The modulation tones of Table 3 may be used, for example, in an opticalnetworking configuration that only supports a range of modulation tonesthat is 130 MHz or less. Although additional modulation tones may fitwithin the supported range, teachings of certain embodiments recognizethat maintaining spacing between the different modulation tonefrequencies may help reduce the ability FWM products near opticalchannels to interfere with the optical channels. For example, in someembodiments, dither generator 140 may increase the laser linewidth by20-30 MHz, and the frequencies of the modulation tones may be separatedto account for dithering of the laser linewidth.

In another example embodiment, modulation tones are applied such that,for at least some three-tone FWM products closest to the opticalchannels, the channels contributing to the three-tone FWM products andthe channels closest to the three-tone FWM products have modulationtones of different frequencies. For example, Table 4, below, lists thethree-tone FWM products, the channels contributing to the three-tone FWMproducts, the channels closest to the three-tone FWM products, and thecrosstalk power of the three-tone FWM products.

Channel Three-tone FWM Closest to FWM Channel Channel Channel ProductFrequency FWM Crosstalk 1 (υ₁) 3 (υ₃) 2 (ν₂) (ν₁ + υ₃ − υ₂) Product (dB)A D B υ_(A) + 325 GHz C −67.1 A D C υ_(A) + 200 GHz B −65.0 A F Bυ_(A) + 700 GHz E −72.3 A F E υ_(A) + 150 GHz B −71.6 A H C υ_(A) + 1100GHz  G −82.1 A H D υ_(A) + 900 GHz F −83.5 A H F υ_(A) + 525 GHz D −84.6A H G υ_(A) + 275 GHz C −79.1 B C A υ_(A) + 475 GHz D −65.4 B C Dυ_(A) + −25 GHz A −68.6 B E A υ_(A) + 900 GHz F −72.6 B E F  υ_(A) + 25GHz A −72.5 B F C υ_(A) + 750 GHz E −66.5 B F E υ_(A) + 325 GHz C −67.8B H E υ_(A) + 850 GHz F −83.0 B H F υ_(A) + 700 GHz E −83.0 C E Bυ_(A) + 850 GHz F −68.7 C E D υ_(A) + 525 GHz D −62.7 C E F υ_(A) + 150GHz B −68.2 C G A υ_(A) + 1425 GHz  H −80.7 C G E υ_(A) + 700 GHz E−74.1 C G H  υ_(A) + 25 GHz A −78.9 D F A υ_(A) + 1375 GHz  H −83.2 D FH υ_(A) + −25 GHz A −85.2 D G E υ_(A) + 900 GHz F −69.2 D G F υ_(A) +750 GHz E −69.5 E F B υ_(A) + 1425 GHz  H −81.5 E F D υ_(A) + 1100 GHz G −68.1 E F G υ_(A) + 475 GHz D −71.2 E F H υ_(A) + 200 GHz B −82.7 F HG υ_(A) + 1150 GHz  G −66.8

Applying the modulation tones of Table 3 to channels A-H may result inmost, but not all, of the three-tone FWM products listed in Table 4having three contributing channels and a closest channel with fourdifferent modulation tones. For example, as shown in Table 4, FWM ofchannels A, D, and B results in a three-tone FWM product having afrequency of v_(A)+325 GHz that is near channel C. Applying themodulation tones of Table 3 to channels A, D, B, and C results in allfour channels having modulation tones of different frequencies.

Applying the modulation tones of Table 3 to channels A-H may result in11 of the FWM products listed in Table 4 having three contributingchannels and a closest channel, two of which share a modulation tonefrequency. For example, as shown in Table 4, FWM of channels A, F, and Bresults in a three-tone FWM product having a frequency of v_(A)+700 GHzthat is near channel E. Applying the modulation tones of Table 3 tochannels A, F, B, and E results in channels A and E sharing the samemodulation tone frequency. Even if this scenario, however, applyingdifferent modulation tone frequencies to channels A, F, and B may stillreduce the ability of the resulting FWM product having a frequency ofv_(A)+700 GHz to interfere with channel E even though channels A and Emay share the same modulation tone frequency.

As explained above, teachings of certain embodiments recognize thatusing modulation tones of different frequencies may reduce or spread outnoise. FIG. 7A illustrates an example plot 700A of the noise floorcharacteristics when every channel in an example channel window has amodulation tone of the same frequency. FIG. 7B illustrates an exampleplot 700B of the noise floor characteristics when the modulation toneshave different frequencies. As can in plot 700A, noise accumulates atthe same frequency (approximately 600 MHz in this example). In plot700B, the noise from each FWM product is spread over a larger range offrequencies, resulting in a broader noise bandwidth and lower peak noisepower density.

FIG. 8 illustrates an example calculated plot 800 of the noise spectrumwithin 600 MHz of channel B for three difference scenarios. In thisexample, channels A, B, and C are spaced such that the FWM productv_(A)+v_(C)−v_(B) is approximately 200 MHz from channel B.

Line 810 shows the noise spectrum near channel B when no modulationtones are applied to channels A, B, and C. Line 820 shows the noisespectrum near channel B when modulation tones of 2.00 GHz are applied tochannels A. B, and C. In this example, applying modulation tones lowersthe noise power at 200 MHz but results in the same sharp curve as line810.

Line 830 shows the noise spectrum near channel B when a modulation toneof 2.05 GHz is applied to channel A, a modulation tone of 2.00 GHz isapplied to channel B, and a modulation tone of 2.08 GHz is applied tochannel C. In this example, applying modulation tones of differentfrequencies to channels A, B, and C effectively reduces the noise peakat 200 MHz and broadens the noise spectrum.

FIG. 9 illustrates an example method 900 for transporting signals over afiber optic network according to one embodiment. At step 910, a firstmodulation tone is generated. At step 920, a second modulation tone isgenerated. In this example, the second modulation tone has a frequencydifferent from the first modulation tone. In some embodiments, tonegenerator 130 generates the first modulation tone and the secondmodulation tone.

At step 930, a first information signal is modulated into a firstoptical signal. In some embodiments, optical modulator 120 modulates thefirst information signal into the first optical signal. At step 940, thefirst modulation tone is applied to the first optical signal. In someembodiments, transmitter 100 applies the first modulation tone to thefirst optical signal to yield modulated optical signal 160.

At step 950, a second information signal is modulated into a secondoptical signal. In some embodiments, optical modulator 120 modulates thesecond information signal into the second optical signal. At step 960,the second modulation tone is applied to the second optical signal. Insome embodiments, transmitter 100 applies the second modulation tone tothe second optical signal to yield modulated optical signal 160.

At step 970, the first optical signal and the second optical signal aremultiplexed into a single multiplexed optical signal. In someembodiments, wavelength multiplexer 220 multiplexes the first opticalsignal and the second optical signal into a single multiplexed opticalsignal. At step 980, the single multiplexed optical signal istransmitted over a fiber optic network. In some embodiments, the singlemultiplexed optical signal is transmitted over optical fiber 230. Atstep 990, the single multiplexed optical signal is demultiplexed. Insome embodiments, wavelength de-multiplexer 240 demultiplexes the singlemultiplexed optical signals.

The steps of method 900 may be performed in any order and may beperformed simultaneously. As one example, step 910 may be performedbefore, during, or after step 930. As another example, step 930 may beperformed before, during, or after step 940.

The present disclosure encompasses all changes, substitutions,variations, alterations, and modifications to the example embodimentsdescribed herein that a person having ordinary skill in the art wouldcomprehend. Similarly, where appropriate, the appended claims encompassall changes, substitutions, variations, alterations, and modificationsto the example embodiments described herein that a person havingordinary skill in the art would comprehend.

What is claimed is:
 1. A system comprising: at least one tone generatoroperable to generate a plurality of modulation tones comprising: a firstmodulation tone having a first modulation frequency; and a secondmodulation tone having a second modulation frequency that is differentfrom the first modulation frequency; a plurality of transmittersoperable to generate a plurality of modulated optical signals using theplurality of modulation tones and a plurality of information signals,each optical signal comprising a primary frequency, the plurality ofmodulated optical signals comprising: a first modulated optical signalgenerated using the first modulation tone; and a second modulatedoptical signal generated using the second modulation tone; and awavelength multiplexer operable to multiplex the plurality of modulatedoptical signals into a single optical fiber; wherein: frequencies of afirst plurality of four-wave mixing (FWM) products caused by the firstmodulation tone are at least a predefined distance away from any of theprimary frequencies of the plurality of optical signals; and frequenciesof a second plurality of FWM products caused by the second modulationtone are at least the predefined distance away from any of the primaryfrequencies of the plurality of optical signals.
 2. The system of claim1, wherein each of the first and second modulation frequencies aregreater than the primary frequencies of the plurality of opticalsignals.
 3. The system of claim 1, wherein the primary frequencies ofthe plurality of optical signals are spaced at one or more selecteddistances apart to reduce the ability of one or more FWM products causedby multiplexing the plurality of optical signals into a single fiber tointerfere with any of the primary frequencies of the plurality ofoptical signals.
 4. The system of claim 1, wherein the plurality ofmodulation tones are suppression tones operable to suppress effects fromstimulated Brillouin scattering (SBS).
 5. A method comprising: receivinga plurality of modulation tones from at least one tone generator, theplurality of modulation tones comprising: a first modulation tone havinga first modulation frequency; and a second modulation tone having asecond modulation frequency that is different from the first modulationfrequency; and generating a plurality of modulated optical signals usingthe plurality of modulation tones and a plurality of informationsignals, each optical signal comprising a primary frequency, theplurality of modulated optical signals comprising: a first modulatedoptical signal generated using the first modulation tone; and a secondmodulated optical signal generated using the second modulation tone;wherein: frequencies of a first plurality of four-wave mixing (FWM)products caused by the first modulation tone are at least a predefineddistance away from any of the primary frequencies of the plurality ofoptical signals; and frequencies of a second plurality of FWM productscaused by the second modulation tone are at least the predefineddistance away from any of the primary frequencies of the plurality ofoptical signals.
 6. The method of claim 5, wherein the one or more FWMproducts comprises two-tone FWM products and three-tone FWM products. 7.The method of claim 5, further comprising multiplexing the plurality ofmodulated optical signals into a single optical fiber.
 8. The method ofclaim 5, wherein each of the first and second modulation frequencies aregreater than the primary frequencies of the plurality of opticalsignals.
 9. The method of claim 5, wherein the primary frequencies ofthe plurality of optical signals are spaced at one or more selecteddistances apart to reduce the ability of one or more FWM products causedby multiplexing the plurality of optical signals into a single fiber tointerfere with any of the primary frequencies of the plurality ofoptical signals.
 10. The method of claim 5, wherein the plurality ofmodulation tones are suppression tones operable to suppress effects fromstimulated Brillouin scattering (SBS).
 11. A system comprising: at leastone tone generator operable to generate a plurality of modulation tonescomprising: a first modulation tone having a first modulation frequency;and a second modulation tone having a second modulation frequency thatis different from the first modulation frequency; and a plurality oftransmitters operable to generate a plurality of modulated opticalsignals using the plurality of modulation tones and a plurality ofinformation signals, each optical signal comprising a primary frequency,the plurality of modulated optical signals comprising: a first modulatedoptical signal generated using the first modulation tone; and a secondmodulated optical signal generated using the second modulation tone;wherein: frequencies of a first plurality of four-wave mixing (FWM)products caused by the first modulation tone are at least a predefineddistance away from any of the primary frequencies of the plurality ofoptical signals; and frequencies of a second plurality of FWM productscaused by the second modulation tone are at least the predefineddistance away from any of the primary frequencies of the plurality ofoptical signals.
 12. The system of claim 11, wherein the one or more FWMproducts comprises two-tone FWM products and three-tone FWM products.13. The system of claim 11, further comprising a wavelength multiplexeroperable to multiplex the plurality of modulated optical signals into asingle optical fiber.
 14. The system of claim 11, wherein each of thefirst and second modulation frequencies are greater than the primaryfrequencies of the plurality of optical signals.
 15. The system of claim11, wherein the primary frequencies of the plurality of optical signalsare spaced at one or more selected distances apart to reduce the abilityof one or more FWM products caused by multiplexing the plurality ofoptical signals into a single fiber to interfere with any of the primaryfrequencies of the plurality of optical signals.
 16. The system of claim11, wherein the plurality of modulation tones are suppression tonesoperable to suppress effects from stimulated Brillouin scattering (SBS).17. The system of claim 11, further comprising at least one dithergenerator operable to broaden the effective linewidth of the pluralityof modulated optical signals by applying one or more dither signals. 18.The system of claim 1, wherein at least some of the plurality ofmodulated optical signals are unequally spaced.
 19. The system of claim1, wherein: adjacent modulated optical signals are generated frommodulation tones of different frequencies; and non-adjacent modulatedoptical signals are generated from one or more modulation tones of thesame frequency.